JP4081731B2 - Switching power supply - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a switching power supply apparatus, and more particularly to a partial resonance switch using a resonance phenomenon.
[0002]
[Prior art]
Conventionally, as a partial resonance technique of a switching power supply in which pulse width control of a fixed frequency is performed, a soft switching circuit (Japanese Patent Laid-Open No. 6-189538) of a current-excited switching power supply previously provided by the present applicant is disclosed. is there. FIG. 4 shows the circuit configuration, and FIG. 5 shows the operation waveform.
[0003]
In the circuit shown in FIG. 4, after the main switch element 102 is turned off, the second switch element 104 is turned off with a delay. If the main switch element 102 is turned off while the second switch element 104 is in an on state, the surge voltage and noise generated at the time of turn-off are suppressed by the snubber capacitor 103.
[0004]
The current discontinuous mode is a method in which the main switch element 102 is turned on after the excitation energy of the transformer has been released. When the excitation energy has been released, the transformer uses the energy of its own voltage to the winding. Until the main switch element 102 is turned on next time, the resonance whose amplitude is the difference between the voltage of the snubber capacitor 103 and the voltage of the DC power supply 111 is the primary winding of the capacitor 103 and the transformer. Wake up by 101.
[0005]
FIG. 5 shows voltage waveforms across the main switch element 102. In the discontinuous period of the waveform diagram shown in FIG. 5, since both the main switch element 102 and the second switch element 104 are in the off state, the resonance stops when the voltage of the snubber capacitor 103 becomes the lowest. The reason is that the current in the discharge direction of the snubber capacitor 103 flows through the parasitic diode of the second switch element 104, whereas the current in the charge direction flows because the second switch element 104 is in the OFF state. Because you can't.
[0006]
When the main switch element 102 is turned on, the voltage of the snubber capacitor 103 is the lowest value. This value is also selected by appropriately selecting the winding ratio of the primary winding 101 and the secondary winding 107 of the transformer. Can be zero. As a result, even if the capacitance of the snubber capacitor 103 is increased, the loss generated at turn-on can be reduced. On the other hand, by increasing the capacitance of the snubber capacitor 103, the loss at the time of turning off the main switch element 102 can be reduced.
[0007]
From the time when the resonance by the snubber capacitor 103 and the primary winding 101 of the transformer ceases until the main switch element 102 is turned on, the short-period resonance by the primary winding 101 of the transformer and the surrounding stray capacitance continues. However, since the stray capacitance is small, even if it is turned on at a high voltage, the loss is small. That is, switching in which both the turn-on and turn-off losses are small is performed.
[0008]
[Problems to be solved by the invention]
The conventional partial resonance method shown above has the following problems when the input voltage is in a wide range. The voltage of the snubber capacitor 103 when the resonance between the snubber capacitor 103 and the primary winding 101 of the transformer stops is a value obtained by subtracting the flyback voltage of the primary winding 101 of the transformer from the input voltage. Therefore, in order for the voltage when the resonance stops to be zero over the entire range of the input voltage, the number of turns so that the flyback voltage of the primary winding 101 of the transformer matches the highest value of the input voltage. The ratio must be chosen.
[0009]
On the other hand, the voltage applied to the main switch element 102 during the OFF period of the main switch element 102 is a value obtained by adding the flyback voltage of the primary winding 101 of the transformer to the input voltage. A high switching element is required. In addition, although it is one of the disadvantages of the current discontinuous mode, a switch element having a large current capacity is required because the peak current of the main switch element 102 is large. That is, it can be said that the cost of the main switch element is increased because the withstand voltage is high and the current capacity is large.
[0010]
The present invention eliminates such drawbacks and can be applied to the continuous current mode, and can be turned on and off without limiting the turns ratio of the primary and secondary windings of the transformer. It is an object of the present invention to provide a switching power supply device that can suppress both losses to a small value.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention controls a main switch element connected in series to a primary winding of a transformer, a snubber capacitor connected in parallel to the main switch element, and an on period of the main switch element. In a switching power supply device having a pulse width control circuit that performs the above operation, a diode is inserted in series with the main switch element, an inductor is inserted in series with the snubber capacitor, and a second diode and a second switch element are connected in parallel with the inductor. A series circuit is connected, and a delay circuit is connected between the control electrode of the second switch element and the pulse width control circuit to delay both the rising and falling times of the pulse by a predetermined time.
[0012]
[Action]
After the main switch element is turned off, the second switch element is turned off with a delay. When the main switch element is turned off, the second switch element is still on, so that the surge voltage at the time of turn-off is absorbed by the snubber capacitor. At this time, the current for charging the snubber capacitor passes through the second switch element, and the current for discharging the snubber capacitor passes through the inductor. After the surge absorption period, the voltage of the snubber capacitor becomes constant at a value obtained by adding the flyback voltage of the primary winding of the transformer to the input voltage. At this time, the second switch element is turned off. Yes.
[0013]
When the main switch element is turned on from the off state, the snubber capacitor flows through the inductor and the main switch element and is discharged. The discharge is half-wave resonant because a diode is connected in series with the main switch element, but when the inductor current returns from zero to a half-sine wave half-cycle again, the snubber capacitor has no polarity. Since the voltage that is inverted and equal to the voltage before the start of discharging is charged, if the second switch element is turned on at this time, the snubber capacitor flows through the transformer primary winding and the second switch element. Discharge.
[0014]
If the switching power supply device is a flyback converter system, the charging energy of the capacitor is temporarily changed to the excitation energy of the transformer, and if it is a forward converter system, it becomes a part of the load current. The loss is zero except for the loss due to the resistance component existing on the charging circuit.
[0015]
A diode connected in series with the second switch element prevents the discharge of the capacitor from flowing through the second switch element when the main switch element is turned on. A diode connected in series with the main switch element prevents the discharge of the capacitor from flowing through the main switch element when the second switch element is turned on.
[0016]
【Example】
FIG. 1 is a circuit diagram showing a flyback converter type switching power supply according to an embodiment of the present invention. 2 and 3 are waveform diagrams showing voltage-current waveforms in the main part of the circuit diagram.
[0017]
In the circuit shown in FIG. 1, the main switch element 12 and the second switch element 4 are repeatedly turned on and off by the pulse voltage output from the pulse width control circuit 14, but the pulse applied to the second switch element 4 is delayed. Delayed by circuit 5. When the main switch element 12 is turned on, the resonance current generated by the snubber capacitor 13 and the inductor 2 flows through the main switch element 12, and the exciting current of the primary winding of the transformer also flows through the main switch element 12. The sum of these two currents is shown by the waveform of ID1 shown in FIG. Further, the voltage and current of the snubber capacitor 13 in the section between the time t1 when the main switch element 12 is turned on and the time t2 when the second switch element 4 is turned on are shown by the waveforms of VR and IR in FIG. .
[0018]
If the period between t1 and t2 is adjusted to a half cycle of resonance between the capacitor 13 and the inductor 2, the voltage of the capacitor 13 becomes a value obtained by adding a negative sign to the value immediately before the resonance at time t2. That is, the voltage is inverted. At time t2, since the second switch element 4 is turned on, the capacitor 13 is discharged through the second switch element. Since this current passes through the primary winding 11 of the transformer, the excitation energy of the transformer increases. That is, the charging energy of the capacitor 13 is converted into the excitation energy of the transformer. ID 2 in FIG. 2 indicates a waveform of a current flowing through the second switch element 4.
[0019]
At time t3 when the main switch element 12 is turned off, the second switch element 4 is kept on, so that the surge voltage generated when the main switch element 12 is turned off is absorbed by the snubber capacitor 13.
[0020]
In the waveform shown in FIG. 2, ID1 and VD1 are the current and voltage of the main switch element 12. At time t1, VD1 is a high value, but since ID1 is nearly zero, no turn-on loss occurs. At time t3, ID1 is a high value, but VD1 is almost zero, so that no turn-off loss occurs. Thus, in the circuit shown in FIG. 1, switching loss does not occur or is a sufficiently small value.
[0021]
No matter what value VD1 takes at t1, ID1 rises from zero, so that no turn-on loss occurs. When operating in the continuous current mode, the main switch element 12 is turned on while the voltage of the main switch element 12 is high. However, since the discharge current of the capacitor 13 rises from zero, no turn-on loss occurs.
[0022]
The embodiment shown in FIG. 1 is a flyback converter type switching power supply, but the present invention can also be implemented in a forward converter type switching power supply.
[0023]
【The invention's effect】
Compared with the conventional method shown in FIG. 4, an inductor and a diode are added, but when implemented under a wide input voltage condition, it can be used in a continuous current mode, so the peak current flowing through the main switch element and the transformer is reduced. The cost of the main switch element and transformer can be reduced. In addition, since it can be applied to both the flyback converter method and the forward converter method, the application range is wide.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a flyback converter switching power supply according to an embodiment of the present invention.
FIG. 2 is a waveform diagram showing waveforms of voltage and current in the main part of FIG.
FIG. 3 is a waveform diagram showing waveforms of voltage and current in the main part of FIG.
FIG. 4 is a circuit diagram showing an example of a conventional method.
5 is a waveform diagram showing waveforms of voltage and current in the main part of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 1st diode 2 Inductor 3 2nd diode 4 2nd switch element 5 Delay circuit 11 Transformer primary winding 12 Main switch element 13 Snubber capacitor 14 Pulse width control circuit 15 Transformer secondary winding 16 Diode 17 Capacitor 18 Load circuit 19 DC power supply 101 Transformer primary winding 102 Main switch element 103 Snubber capacitor 104 Second switch element 105 Delay circuit 106 Pulse width control circuit 107 Transformer secondary winding 108 Diode 109 Capacitor 110 Load circuit 111 DC power supply VG1 Main switch element gate voltage VG2 Second switch element gate voltage ID1 Main switch element current ID2 Second switch element current VD1 Main switch element voltage VR Voltage of snubber capacitor 13 Snubber capacitor 1 Current of

Claims (1)

A primary winding of a transformer, a main switch element connected in series to the primary winding, a snubber capacitor connected in parallel to the main switch element, and a pulse width control circuit for controlling an on period of the main switch element In the switching power supply device provided, a first diode is inserted in series with the main switch element, an inductor is inserted in series with the snubber capacitor, and a second diode and a second switch element are formed in parallel with the inductor. A delay circuit connected to the series circuit and delayed by a predetermined time from the turn-on time and turn-off time of the main switch element to give a turn-on and turn-off control signal to the second switch element is provided with the pulse width control circuit and the first A switching power supply device, characterized in that the switching power supply device is connected between control electrodes of two switch elements.

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